Chirped pulse amplification (CPA) systems are used to amplify ultra-short laser pulses to high power levels. In the CPA system, an input laser pulse is stretched out in time, amplified, and then compresses after amplification. The former Nova laser at the Lawrence Livermore National Laboratory was an example of a laser utilizing chirped pulse amplification and operating at the petawatt level. In addition to research lasers, some commercial Ti:sapphire lasers utilize CPA to generate peak powers greater than a gigawatt.
CPA laser systems utilize the temporal and spreading of the input pulse prior to amplification to reduce the damage that can occur in gain medium through nonlinear processes such as self-focusing as optical power increases in the amplifier section. Typically, the stretcher operates so that the low-frequency components of the laser pulse travel a shorter path than the high-frequency components. After passing through the stretcher, the laser pulse becomes positively chirped (i.e., the high-frequency components lag behind the low-frequency components). Utilizing a stretcher, it is possible to increase the duration of the input pulse by one or more orders of magnitude. The stretched pulse, with a lower intensity, is then amplified using one or more amplification stages. After amplification, the amplified laser pulse is temporally compressed using a compressor, preferably back to the original pulse width.
The presence of optical material in a chirped pulse amplification laser system causes dispersion in the chirped pulse, which must be compensated somewhere else in the system before the pulse can be compressed to a minimum pulse length. Several techniques exist which have been used for dispersion compensation including: the use of an acousto-optic programmable dispersive filter (AOPDF) and a spatial light modulator (SLM). The AOPDF is programmable and adaptive, however this option is costly and has to be actively controlled for the laser system to operate properly. The SLM, for example, a liquid crystal SLM, is also programmable and adaptive, however this option suffers from discreet “pixels” of adjustable phase delay as well as difficulty during calibration.